gs0510 / ocaml-rs

OCaml extensions in Rust

ocaml-rs - OCaml extensions in Rust

ocaml-rs allows for OCaml extensions to be written directly in Rust with no C stubs. It was originally forked from raml, but has been almost entirely re-written thanks to support from the OCaml Software Foundation.

Works with OCaml versions 4.06.0 and up

Please report any issues on github

Getting started

Take a look at the ocaml-rust-starter project for a basic example to help get started with ocaml-rs.

On the Rust side, you will need to add the following to your Cargo.toml:

ocaml = "*"

or

ocaml = {git = "https://github.com/zshipko/ocaml-rs"}

For macOS you will need also to add the following to your project's .cargo/config file:

[build]
rustflags = ["-C", "link-args=-Wl,-undefined,dynamic_lookup"]

This is because macOS doesn't allow undefined symbols in dynamic libraries by default.

Additionally, if you plan on releasing to opam, you will need to vendor your Rust dependencies to avoid making network requests during the build phase, since reaching out to crates.io/github will be blocked by the opam sandbox. To do this you should run:

cargo vendor

then follow the instructions for editing .cargo/config

Features

• derive
  • enabled by default, adds #[ocaml::func] and friends and derive implementations for FromValue and ToValue
• link
  • link the native OCaml runtime, this enables ocaml::runtime::init, which is equivalent to caml_main
• no-std
  • Allows ocaml to be used in #![no_std] environments like MirageOS

Documentation

https://docs.rs/ocaml

Examples

// Automatically derive `ToValue` and `FromValue`
#[derive(ocaml::ToValue, ocaml::FromValue)]
struct Example<'a> {
    name: &'a str,
    i: ocaml::Int,
}


#[ocaml::func]
pub fn incr_example(mut e: Example) -> Example {
    e.i += 1;
    e
}

#[ocaml::func]
pub fn build_tuple(i: ocaml::Int) -> (ocaml::Int, ocaml::Int, ocaml::Int) {
    (i + 1, i + 2, i + 3)
}

#[ocaml::func]
pub fn average(arr: ocaml::Array<f64>) -> Result<f64, ocaml::Error> {
    let mut sum = 0f64;

    for i in 0..arr.len() {
        sum += arr.get_double(i)?;
    }

    Ok(sum / arr.len() as f64)
}

// A `native_func` must take `ocaml::Value` for every argument and return an `ocaml::Value`
// these functions have minimal overhead compared to wrapping with `func`
#[ocaml::native_func]
pub fn incr(value: ocaml::Value) -> ocaml::Value {
    let i = value.int_val();
    ocaml::Value::int(i + 1)
}

// This is equivalent to:
#[no_mangle]
pub extern "C" fn incr2(value: ocaml::Value) -> ocaml::Value {
    ocaml::body!((value) {
        let i = value.int_val();
        ocaml::Value::int( i + 1)
    })
}

// `ocaml::native_func` is responsible for:
// - Ensures that #[no_mangle] and extern "C" are added, in addition to wrapping
// - Wraps the function body using `ocaml::body!`

// Finally, if your function is marked [@@unboxed] and [@@noalloc] in OCaml then you can avoid
// boxing altogether for f64 arguments using a plain C function and a bytecode function
// definition:
#[no_mangle]
pub extern "C" fn incrf(input: f64) -> f64 {
    input + 1.0
}

#[cfg(feature = "derive")]
#[ocaml::bytecode_func]
pub fn incrf_bytecode(input: f64) -> f64 {
    incrf(input)
}

The OCaml stubs would look like this:

type example = {
    name: string;
    i: int;
}

external incr_example: example -> example = "incr_example"
external build_tuple: int -> int * int * int = "build_tuple"
external average: float array -> float = "average"
external incr: int -> int = "incr"
external incr2: int -> int = "incr2"
external incrf: float -> float = "incrf_bytecode" "incrf" [@@unboxed] [@@noalloc]

For more examples see test/src or ocaml-vec.

Type conversion

This chart contains the mapping between Rust and OCaml types used by ocaml::func

Rust type	OCaml type
()	unit
isize	int
usize	int
i8	int
u8	int
i16	int
u16	int
i32	int32
u32	int32
i64	int64
u64	int64
f32	float
f64	float
str	string
String	string
Option<A>	'a option
Result<A, B>	exception
(A, B, C)	'a * 'b * 'c
&[Value]	'a array (no copy)
Vec<A>, &[A]	'a array
BTreeMap<A, B>	('a, 'b) list
LinkedList<A>	'a list

Even though &[Value] is specifically marked as no copy, a type like Option<Value> would also qualify since the inner value is not converted to a Rust type. However, Option<String> will do full unmarshaling into Rust types. Another thing to note: FromValue for str and &[u8] is zero-copy, however ToValue for str and &[u8] creates a new value - this is necessary to ensure the string is registered with the OCaml runtime.

If you're concerned with minimizing allocations/conversions you should use Value type directly.

Upgrading

Since 0.10 and later have a much different API compared to earlier version, here is are some major differences that should be considered when upgrading:

• FromValue and ToValue have been marked unsafe because converting OCaml values to Rust and back also depends on the OCaml type signature.
  • A possible solution to this would be a cbindgen like tool that generates the correct OCaml types from the Rust code
• ToValue now takes ownership of the value being converted
• The caml! macro has been rewritten as a procedural macro called ocaml::func, which performs automatic type conversion
  • ocaml::native_func and ocaml::bytecode_func were also added to create functions at a slightly lower level
  • derive feature required
• Added derive implementations for ToValue and FromValue for stucts and enums
  • derive feature required
• i32 and u32 now map to OCaml's int32 type rather than the int type
  • Use ocaml::Int/ocaml::Uint to refer to the OCaml's int types now
• Array and List now take generic types
• Strings are converted to str or String, rather than using the Str type
• Tuples are converted to Rust tuples (up to 20 items), rather than using the Tuple type
• The core module has been renamed to sys and is now just an alias for the ocaml-sys crate and all sub-module have been removed